Exploring the sea surface temperature updating in WRF model over northwestern Mediterranean Sea

July 2019 in Catalonia, northeastern Spain, was an anomalously warm month marked by an irregular precipitation pattern, both spatially and temporally. Throughout this period, some discrepancies between WRF operational forecasts and observations were detected, which might stem from the lack of the sea surface temperature (SST) updating in the WRF model configuration. To study the SST-updating effects on WRF v4.5, two simulations were performed using ERA5 as initial and boundary conditions, the first one without updating SST and the second one updating it. Specifically, the study focused on two distinct periods of July 2019: days 8-10, characterized by storms over the Pyrenees (hereafter STORM), and days 24-26, characterized by a heatwave (hereafter HEATW). The objectives of this study are 1) to assess the SST updating impacts on WRF model, particularly on the surface layer and planetary boundary layer (PBL) parametrizations, and 2) to explore the differential behavior of the WRF model between storm and heatwave conditions.              

Results show that changes in SST modifies surface layer parameterization through surface fluxes, with the latent heat flux being more sensitive than the sensible heat flux. These fluxes impacted the stability regimes through the modification of the Monin-Obukhov length and the stability functions. During STORM period the atmosphere tends towards neutrality when updating SST, hence is dominated by wind shear turbulence, whereas on HEATW period the SST updating leads to very unstable conditions, dominated by buoyancy.

Regarding the PBL parameterization, the SST updating leads to an increase of the PBL height during STORM and a decrease during HEATW, with a greater absolute variation observed during the latter period. Averaged potential temperature vertical profiles at 00 UTC reveal greater variability during storm conditions, especially at higher levels (500-1000 m), whereas during heatwave situations, the surface mixing layer is discernible up to ~80 m, corresponding to the maximum mixing height. Furthermore, during HEATW the averaged potential temperature profile revealed the maximum differences between the updated and non-updated simulations at the surface, with differences decreasing with height. Conversely, during STORM, the higher differences between profiles were located a few tens of meters above the surface.

These findings show the significance of SST updating in operational forecasting during July 2019. Specifically, during the period of storm, the SST updating presents a greater impact on meteorological variables, particularly on air potential temperature, compared to the period of heatwave. These results are specific to July 2019 over the northwestern Mediterranean Sea, and further investigation is needed to assess their applicability in different time periods with similar meteorological conditions. Despite that, the study helps to a better understanding of the sensitivity of the WRF model to SST changes and the need of a proper SST representation.